This invention relates generally to techniques for drying lacquer, ink and other coatings applied to the outer surface of cylindrical metal cans, and more particularly to apparatus for rapidly and efficiently drying such coatings by the direct application of a gas flame thereto.
Metal cans for beverages, food and other products are now mass-produced at exceptionally high rates. Many commercially-produced cans are fabricated by drawing and ironing a metal blank formed of aluminum or steel in a multiphase operation, thereby avoiding the need for seaming. But before the cans are filled and a lid bonded thereto, it is customary to protectively coat and decorate the outer surface thereof. Such coatings are usually in three layers, the first being a base coat which is a uniform white or ground color. The second layer is the print coat in which printed and decorative matter is laid over the base coat. (In some instances, the print coat is applied directly to the metal can.) Finally, a third layer is applied, whose purpose is to protect the print layer, this overcoat being a clear lacquer.
The coatings are generally of the lacquer type which dry or cure by evaporation of the volatile components. The film-forming constituent is usually a high molecular weight polymer, such as a polyester. Other types of lacquer coatings are based on acrylic resins. The solvents used are generally of the low-boiling type, such as aromatics.
The term "coating" as used herein is intended to encompass any protective lacquer, decorative layer, printing ink or any other material applied in the wet state on the surface of the can.
It is essential, after each coating is laid down on the can surface, that it be dried to specifications. In some instances, the coatings must be thoroughly dried, in which event even the slightest tackiness is objectionable. In other cases, some residual wetness is acceptable. To this end, it has heretofore been the practice in order to achieve a high-production rate, to convey a continuous train of cans on a conveyor chain through an elongated oven in which the chain travels up and down through a sinuous path to lengthen the exposure time of the cans to the heat without unduly extending the length of the oven.
The heat supplied to such ovens is in indirect form in that air is first heated by a gas-flame burner external to the oven chamber and then blown into the oven at an elevated temperature sufficient to dry the fresh coatings on the cans passing therethrough but well below the ignition point of the coatings. The drying time is, of course, a function of the oven temperature.
One must be careful to maintain the temperature within the oven at a level below which the coating will catch fire. Moreover, since the cans remain in the oven for a relatively long period, one must be careful that the oven temperature is not so high as to anneal the cans when aluminum is used as the can metal. On the other hand, the volatilized solvents exhausted from the oven cannot be discharged directly into the atmosphere, for they may be noxious and highly polluting. Hence with existing ovens, it is the practice to provide after-burners to burn off or precipitate the solvent vapors.
The existing indirect-heating technique for drying can coatings is inherently inefficient and wasteful in terms of energy expenditure. The heat to volatilize the solvents is derived from the heated atmosphere of the oven, whereas the heat to elevate the temperature of this atmosphere is derived from a gas-flame heater. Only a fraction of the thermal energy supplied by the gas-flame heater is exploited, for most of this energy is dissipated in heating a huge volume of air. Thus with one known type of oven for drying can coatings, as much as 4 million BTU's of thermal energy are expended in the course of one hour, only a portion of which actually serves to carry out the can drying operation.
The sharply rising cost of combustible gas renders the operation of existing types of drying ovens increasingly expensive. But this is not the only objectionable factor, for the scarcity of combustible gas makes it vital to conserve the available supply thereof, and an oven wasteful of gas cannot be tolerated. Furthermore, because gas is currently in short supply, one dare not expand existing can-drying facilities, for fear that gas may not be available for the enlarged operation.
It would therefore appear to be distinctly advantageous from the standpoint of the efficient utilization of the available supply and to best conserve energy to apply the flame directly to the surface of the cans, rather than to air which in turn serves to dry the can surface. But it has not heretofore been possible to use a direct-flame technique for drying wet coatings, for this would result in ignition and in uncontrolled burning resulting in the destruction of the coatings.